There is currently no specific clinical test or procedure that can definitively diagnose Alzheimer's disease. Alzheimer's disease is therefore considered a diagnosis of exclusion - in other words, the many other possible causes of dementia must be ruled out by thorough evaluation before the clinical diagnosis of Alzheimer's disease can be made. To do this, the doctor will:

order various laboratory tests and usually some form of imaging procedure

The reason a thorough evaluation is so important is that sometimes other treatable causes of dementia are detected. For example, although Alzheimer's disease cannot be diagnosed by a specific laboratory test, laboratory testing may reveal another cause (e.g., thyroid problems) of symptoms similar to those of Alzheimer's disease.

There are 4 major diagnostic imaging techniques that can be used in the process of diagnosis:

computerized tomography (CT)

magnetic resonance imaging (MRI)

single photon emission computerized tomography (SPECT)

positron emission tomography (PET)

None of these can provide a definitive diagnosis, but all of them can help. Although each technique can yield images consistent with Alzheimer's disease, the real value of these tests is the ability to diagnose other causes of dementia that can mimic Alzheimer's disease. For example, an MRI scan may establish that symptoms are due to a blood clot.

The use of genetic tests to diagnose Alzheimer's disease is limited to inherited, early-onset forms of the disease. Testing for late-onset Alzheimer's disease is not recommended, since testing cannot predict who will develop Alzheimer's disease and who will not.

Ultimately, a definitive diagnosis of Alzheimer's disease requires confirmation of the diagnosis by brain biopsy or autopsy. So, a diagnosis by your doctor is a "probable" rather than a "definitive" one.